A device disclosed in patent document 1 and the like is known as a bidirectional power supply device for supplying and receiving power bidirectionally between two DC (Direct Current) power supplies. FIG. 13 is a block circuit diagram of a conventional bidirectional power supply device. Bidirectional DC-DC converter 101 is connected between battery 102a of power supply 102 and capacitor 103a of power supply 103. FIG. 13 shows an example of an automobile power supply, where capacitor 103a is connected to power generator 104 coupled to an engine (not shown). The braking energy of the automobile is first stored in capacitor 103a that can be rapidly charged/discharged, and then charged in battery 102a through DC-DC converter 101. Thus, the braking energy of the automobile is regenerated as power.
Capacitor 103a is connected to first MOS transistor 111, and second MOS transistor 112 is connected in series with MOS transistor 111. MOS transistors 111 and 112 respectively have body diodes 113 and 114 in parallel.
One end of coil 115, which is an inductance element, is connected to a node of MOS transistor 111 and MOS transistor 112, and the other end of coil 115 is connected to battery 102a by way of current sensor 116. Control circuit 117 controls ON/OFF of MOS transistors 111 and 112 in response to the output of current sensor 116.
The voltage of capacitor 103a storing the braking energy of the automobile as power is higher than the voltage of battery 102a at first. Therefore, DC-DC converter 101 performs the step-down operation. The step-down operation normally starts in a soft start.
FIG. 14A to FIG. 14C describe the step-down operation of DC-DC converter 101. FIG. 14A shows ON/OFF operation of first MOS transistor 111, FIG. 14B shows ON/OFF operation of second MOS transistor 112, and FIG. 14C shows change over time of current Iout to battery 102a. DC-DC converter 101 starts the operation at time t0. As shown in FIG. 14A, control circuit 117 gradually increases the ON time of MOS transistor 111 from near zero for the soft start. If MOS transistor 111 and MOS transistor 112 are alternately turned ON/OFF during the soft start, the on-duty of MOS transistor 112 becomes too long, so that the overcurrent may flow from battery 102a to MOS transistor 112. In order to avoid this, MOS transistor 112 is remained turned OFF during the soft start, as shown in FIG. 14B.
Current Iout to battery 102a gradually increases as shown in FIG. 14C, and reaches threshold value Th1 at time t1. Thereafter, MOS transistor 111 and MOS transistor 112 are alternately turned ON/OFF in a steady-state operation. The power of capacitor 103a is thereby charged in battery 102a, and the braking energy is effectively regenerated.
The step-down operation from capacitor 103a to battery 102a has been described in the above description, but a step-up operation is necessary if the voltage of capacitor 103a is lower than the voltage of battery 102a. In the case of the step-up operation, a circuit configuration in which coil 115 is connected to capacitor 103a side is adopted. The step-up operation is realized with a control similar to the step-down operation other than that the timing charts of MOS transistor 111 and MOS transistor 112 are interchanged. A bidirectional power supply device capable of performing the step-down operation and the step-up operation is thus provided.
The overcurrent of MOS transistor 112 is prevented in the above manner. However, since MOS transistor 112 remains turned OFF, current flowing through body diode 114 in forward direction generates every time MOS transistor 111 turns OFF the current flowing to battery 102a through coil 115 during the soft start. Body diode 114 may be overheated by the generated forward current. The present invention provides a bidirectional power supply device in which the body diode does not generate heat.
[Patent document 1] Japanese Patent No. 3,501,226